Here are 2 photos showing how air flies over common sedan and hatchback:
As you'll be able to see, the car window on a sedan faces flow of air on the skin. That flow of air is employed to push raindrops down the rear glass, effectively clearing it from raindrops. On the opposite hand, behind a hatchback, the rear glasss faces quite a vacuum and so flow of air cannot clear raindrops from the car window. Instead, as you aforementioned, hatchbacks deploy rear wipers to clear the raindrops.
